reftable/stack: accept insecure random bytes

The reftable library uses randomness in two call paths:

  - When reading a stack in case some of the referenced tables
    disappears. The randomness is used to delay the next read by a
    couple of milliseconds.

  - When writing a new table, where the randomness gets appended to the
    table name (e.g. "0x000000000001-0x000000000002-0b1d8ddf.ref").

In neither of these cases do we need strong randomness.

Unfortunately though, we have observed test failures caused by the
former case. In t0610 we have a test that spawns a 100 processes at
once, all of which try to write a new table to the stack. And given that
all of the processes will require randomness, it can happen that these
processes make the entropy pool run dry, which will then cause us to
die:

    + test_seq 100
    + printf %s commit\trefs/heads/branch-%s\n
    68d032e9edd3481ac96382786ececc37ec28709e 1
    + printf %s commit\trefs/heads/branch-%s\n
    68d032e9edd3481ac96382786ececc37ec28709e 2
    ...
    + git update-ref refs/heads/branch-98 HEAD
    + git update-ref refs/heads/branch-97 HEAD
    + git update-ref refs/heads/branch-99 HEAD
    + git update-ref refs/heads/branch-100 HEAD
    fatal: unable to get random bytes
    fatal: unable to get random bytes
    fatal: unable to get random bytes
    fatal: unable to get random bytes
    fatal: unable to get random bytes
    fatal: unable to get random bytes
    fatal: unable to get random bytes

The report was for NonStop, which uses OpenSSL as the backend for
randomness. In the preceding commit we have adapted that backend to also
return randomness in case the entropy pool is empty and the caller
passes the `CSPRNG_BYTES_INSECURE` flag. Do so to fix the issue.

Reported-by: Randall S. Becker <[email protected]>
Signed-off-by: Patrick Steinhardt <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

reftable/stack.c

@@ -493,7 +493,7 @@ static int reftable_stack_reload_maybe_reuse(struct reftable_stack *st,
 		close(fd);
 		fd = -1;
 
-		delay = delay + (delay * rand()) / RAND_MAX + 1;
+		delay = delay + (delay * git_rand(CSPRNG_BYTES_INSECURE)) / UINT32_MAX + 1;
 		sleep_millisec(delay);
 	}
 
@@ -659,7 +659,7 @@ int reftable_stack_add(struct reftable_stack *st,
 static int format_name(struct reftable_buf *dest, uint64_t min, uint64_t max)
 {
 	char buf[100];
-	uint32_t rnd = (uint32_t)git_rand(0);
+	uint32_t rnd = git_rand(CSPRNG_BYTES_INSECURE);
 	snprintf(buf, sizeof(buf), "0x%012" PRIx64 "-0x%012" PRIx64 "-%08x",
 		 min, max, rnd);
 	reftable_buf_reset(dest);